This invention relates to the field of radio frequency radiation antenna devices, and particularly to an ultra-wideband (UWB) transverse electromagnetic (TEM)-mode horn antenna.
Ultra-wideband TEM horn antenna designs have been available for fifteen years or more, and are used by the military and others for applying pulsed electromagnetic radiation. Background discussions of TEM horn antenna characteristics are to be found in the papers by Evans, S., and Kong, F.N., "TEM Horn Antenna: Input Reflection Characteristics in Transmission", Proc. IEEE, Vol. 130H, Oct. 1983, pp. 403-409; and by Kerr, J. L., "Short Axial Length Broad-Band Horns" Trans. IEEE, Vol. AP-21, Sep. 1973, pp. 403-409.
In conventional TEM horn antenna design, single sources which offer the highest powers are used to drive single TEM horns. In some designs, multiple-phased, lower power sources drive arrays of horns, giving one source per antenna aperture. However, the path lengths from feed to aperture in the Electric field (E-plane) are not equal, giving rise to large phase error at all but the lowest frequencies, and resulting in low gain and directivity. The only way to improve this without a fundamental design change, is to make the horn much longer in length than it normally would be; which is impractical, expensive and cumbersome when large apertures are required.
It is therefore a principal object of this invention to provide an ultra-wideband TEM planar transmission-line-array horn antenna which is relatively compact for its directivity, and exhibits high-gain, directivity and acceptable losses. The invention is a high-gain, UWB, transverse electromagnetic (TEM) mode parallel-plate planar transmission-line-array horn antenna, utilizing a highly novel binary-tree based design to extend the effective length of antenna. High-power, UWB, radio-frequency electromagnetic pulses are input to the antenna on a two-conductor parallel-plate transmission line which propagates the pulses in the fundamental TEM mode. The signals enter the feed region and then pass to a series Tee parallel-plate, open transmission-line junction. The signal is divided into two signals at the Tee junction, which are then re-directed around curves at approximate 110 deg. bends, and further divided at paralleled Tees into a multiple number of paralleled signals. Each of the signals is conducted down a path of gently flared parallel plates forming a horn, to exit at a radiation aperture. The preferred embodiment utilizes two stages to form a binary tree, parallel-plate, transmission line configuration having four paralleled apertures. However, it is possible to utilize more than two stages, resulting in a larger multiple number of paralleled apertures.
The invention structure produces an equal path length for signals in each of the branches, virtually eliminating phase error in the E-Plane, and producing high gain characteristics over most of the desired frequency range.